[A-AS09] Atmospheric Chemistry

[AAS09-P18] Reevaluation of UV absorption of saturated fatty acids and its implications for atmospheric chemistry in the troposphere

*Shota Saito¹, Naoki Numadate², Hidemasa Teraoka¹, Shinichi Enami², Hirokazu Kobayashi¹, Tetsuya Hama¹ (1.The University of Tokyo, 2.University of Tsukuba)

Keywords:fatty acid, photochemical reaction, volatile organic compounds, laboratory experimental research

Saturated fatty acids are typical organic compounds emitted by living organisms and are abundant and important components of ocean surfaces and marine aerosols. Recently, the photochemical reactions of these saturated fatty acids have attracted much attention in atmospheric chemistry. Although saturated fatty acids have been considered inert to ultraviolet light (wavelength λ > 295 nm), an experimental laboratory study reported that a photochemical reaction triggers the release of volatile organic compounds (alkenes, saturated aldehydes, etc.) [1]. After this experimental study, photochemical reactions of fatty acids have been regarded as a new source of volatile organic compounds in the troposphere [2-4]. In order to quantify this photochemical reaction of fatty acids and estimate its impact on the Earth's atmosphere, it is necessary to accurately quantify the photoabsorption of saturated fatty acids. However, the photoabsorption cross sections of saturated fatty acids at wavelengths longer than 250 nm and their physicochemical origin have remained unresolved to date since the first report in 1931 [5]. In this study, to accurately quantify the photoabsorption cross sections of fatty acids, a typical saturated fatty acid, nonanoic acid (a 9-carbon linear saturated fatty acid), was purified by a recrystallization method to accurately measure the photoabsorption spectrum of pure nonanoic acid.
To purify the nonanoic acid, we recrystallized the nonanoic acid 15 times using our originally developed recrystallization apparatus. To quantitatively measure the optical photoabsorption cross section of the purified nonanoic acid over a wide wavelength range of 190-310 nm, in addition to the usual measurement in a quartz cell, a liquid film method was used in the short wavelength region (190-240 nm) where absorption is strong. UV absorption spectra were measured by changing the optical path length by a factor of 9000 over a wide range from 0.01 mm to 90 mm using this method.
As a result of measuring the absorption spectrum of the purified nonanoic acid, the absorption at 250-310 nm disappeared, revealing that the previously reported photoabsorption of nonanoic acid was due to a trace amount (at most 0.1%) of impurities in the reagent (Fig. 1). We calculated the upper limit of the photoabsorption cross section from the absorption spectrum of the purified nonanoic acid, and determined the value to be 1.3 x 10^-23 cm² (at 295 nm). The photoabsorption cross section of nonanoic acid and the tropospheric solar flux indicated that the photolysis rate of nonanoic acid in the troposphere is at most 1.0 x 10^-9 s^-1, which indicates that the photoabsorption cross section and photolysis rate of nonanoic acid are sufficiently weak. Analysis of the nonanoic acid reagent by nuclear magnetic resonance spectroscopy also revealed that the ketone contained as an impurity affected the photoabsorption.
The present results indicate the possibility that previous photoreaction experiments of saturated fatty acids based on the troposphere (wavelength λ > 295 nm) are contaminated with impurities, indicating the need to reinterpret the experimental results [6].

[1] S. Rossignol et al., Science, 353, 699 (2016).
[2] T. H. Bertram et al., Chem. Soc. Rev.. 47, 2374-2400 (2018).
[3] G. A. Novak et al., Acc. Chem. Res., 53, 1014-1023 (2020).
[4] N. Numadate, J. Phys. Chem. Lett., 13, 8290-8297 (2022).
[5] O. Hartleb et al., Strahlentherapie, 39, 442 (1931).
[6] Saito et al., Sci. Adv., 9, 38, eadj6438. (2023)